The present invention relates generally to an improved system for preparing highly adherent/coherent coatings of high molecular weight amorphous fluorinated polymers such as fluorinated ethylenic-cyclo oxyaliphatic substituted ethylenic copolymers upon substrate surfaces, and more specifically to a method for pre-conditioning the substrate surface for acceptance of a coating of castable high molecular weight amorphous fluorinated polymer layers, specifically a fluoropolymer having a structural formula with the repeating unit: ##STR2## wherein "m" and "n" represent integers providing a molecular weight of between about 230,000 and 500,000, with the relative values of "m" and "n" providing a copolymer with a glass transition temperature ranging from between about 160.degree. C. and 240.degree. C.
In the past, certain fluoropolymers of relatively lower molecular weight and consisting essentially of tetrafluoroethylene have been utilized in a wide variety of applications. Specifically, these polymers have been available in solid or powder form, for use in preparing molded and/or extruded articles as well as films and coatings. For example, the polymer has also been available in aqueous dispersion form for the preparation of non-stick coatings as well as in certain other applications requiring surfaces with low friction. Because of its relatively low coefficient of friction, the material has further been utilized as a finish for multi-filament filaments and/or fibers. One of the most common applications of tetrafluoroethylene polymers has been in the formation of finishes and/or films for cookware to provide non-stick surfaces for food preparation.
Recently, a variety of fluoropolymers, specifically a fluoropolymer having a structural formula with the repeating unit: ##STR3## wherein "m" and "n" represent integers providing a molecular weight of between about 230,000 and 500,000, with the relative values of "m" and "n" providing a copolymer with a glass transition temperature ranging from between about 160.degree. C. and 240.degree. C., a commercial form of which is identified by the code name "Teflon AF Amorphous Fluoropolymer", this material being available commercially from E. I. DuPont de Nemours Co., Inc. of Wilmington, Del. This fluoropolymer has somewhat different physical, electrical, and chemical properties from conventional tetrafluoroethylene polymers, and is suitable for use as a (dispersion, solvent, liquid, or whatever forms are available). In liquid dispersion form, the material has been found adapted to produce castable coatings. Additionally, the material can be hot-pressed into sheets to form useful objects. Because of the unusual physical properties of this high molecular weight amorphous fluoropolymer material, adhesion of the castable materials has presented problems in finding techniques for application primarily because of the unavailability of suitable techniques for achieving adhesion to substrate surfaces. One particularly valuable property of the material is its dielectric constant, with the material having been found to possess the lowest dielectric constant of plastics, particularly at gigahertz frequencies.
In accordance with the present invention, however, a method has been developed which provides a means for laying down a castable film of amorphous fluoropolymer consisting essentially of evaporatively depositing an initial or primer layer of this high molecular weight fluoropolymer (ethylenic-cyclo oxyaliphatic substituted ethylenic) and thereafter applying one or more relatively thick layers of this same material onto the primer layer. In this fashion, the second application is undertaken with the polymer in liquid solution and/or dispersion form, and it is in this form that the castable film develops good adhesion to the surface upon which it is deposited. Also, in this form, the unusual electrical properties are preserved, specifically the low dielectric constant at gigahertz frequencies.
Specifically, and in accordance with the present invention, an initial or primary layer of high molecular weight fluoropolymer of ethylenic-cyclo oxyaliphatic substituted ethylenic is applied to the substrate upon which the castable coating is to be formed. The initial or primer film is applied to the surface of the substrate by vacuum deposition techniques, with such techniques having been found to provide films with good adhesive as well as cohesive properties. Once the primer film has been evaporatively deposited onto the surface of the substrate, a coating of the castable film of high molecular weight amorphous fluorinated copolymer consisting of the same fluoropolymer is applied or otherwise cast thereover. Application techniques for the liquid forms include brushing, spraying, dipping and spinning. Following the casting operation and evaporation of the solvent, the coating formed by the residue possesses the unusual physical and electrical properties of the high molecular weight amorphous fluoropolymer material, and in addition may be deposited as a relatively thick layer onto the surface of the substrate. These coatings, which may be cured with only room temperature or low heat requirements, may have thicknesses ranging up to and over 0.25 millimeters. It has also been found that the coatings, when cast into thin-film form, are substantially pinhole-free, and will achieve this property without requiring sintering.
The castable fluoropolymer (ethylenic-cyclo oxyaliphatic substituted ethylenic) material utilized in accordance with the present invention is available in particulate or granular form. As indicated hereinabove, the molecular weight of the polymer ranges from between about 230,000 and 500,000, and as such, possesses a property of forming stable solutions which can be used to form desirable coatings. For example, the material possesses solubility of up to between about 3% and 5% in certain liquid fluorocarbons, with certain forms of the high molecular weight amorphous material possessing a solubility limit of up to about 12% in such solvents. The technique of the present invention widens the scope of use of the material, since it renders it possible to form relatively thick coatings of this material with the desirable adhesive and cohesive properties. In solution form, the material can be loaded with finely powdered silica, with such materials drying so as to form filled coatings.
Therefore, it is a primary object of the present invention to provide an improved process for the preparation of castable coatings of amorphous fluoropolymers consisting essentially of high molecular weight fluorinated (ethylenic-cyclo oxyaliphatic substituted ethylenic) copolymer and wherein the coatings may be prepared in part by dipping, spinning, brushing, spraying or other application techniques.
It is a further object of the present invention to provide an improved method for the preparation of relatively thick coatings of amorphous fluoropolymers consisting essentially of fluorinated (ethylenic-cyclo oxyaliphatic substituted ethylenic) copolymer, and wherein the initial or primer film is formed upon the substrate by evaporative deposition techniques.
It is yet a further object of the present invention to provide an improved amorphous fluoropolymer coating which is relatively thick in cross-section, and which is repairable in the event of damage to certain portions of the surface thereof.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawing.